Operation of alternating current dynamo electric machines



Nov. 10, 1931. J. l. HULL OPERATION OF ALTERNATING CURRENT DYNAMOELECTRIC MACHINES Filed Nov. 20, 1929 figs. M I 17 MIll/ll/IIIIl/IIIIl/l/l/l/l/I/l Willi/M '7 T c" I" c U j/ PHASE PHASEE WW v 20 Inventor: John LHuH',

b ddk PHASEZ y i His Attorney Patented Nov. 10, 1931 UNITED STATESPATENT OFFICE JOHN I; HULL, OF SCHENECTADY, NEW YORK, ASSIGNORa TOGENERAL ELECTRIC COM- PANY, A CORPORATION OF NEW YORK OPERATION OFALTERNATING CURRENT D-YNAMO ELECTRIC MACHINES Application filed November20, 1929. Serial No. 408,585.

My invention relates to a novel arrangement for neutralizing undesirabletorques that may exist in commutator type dynamo electric machines, asfor example, in commutator type frequency changers.

In industry there exists a large number of types of alternating currentmachinery which can be most advantageously operated at fre quenciessubstantially below the commercial frequencies of 25 and 60 cycles. Afew examples of such machinery are induction motors used for threadingin operations atpaper mills and other places and also vibrating devicesused to operate sand sifters, ore sifters,

and the like. The large and varied nature of the apparatus to beoperated at the low frequencies, together with the needs of variousinstallations, led to the use of both exter I nally propelled andself-propelled frequency changers with many different ratings offrequency, voltage, current and kilowatt capacity. The large actual andpotential demand for such various types of low frequency changers makesit desirable to employ means to neutralize undesirable torques that tendto develop in the commutated winding and overcome commutationdifliculties in the commutated winding. The importance of overcomingcommutation difficulties and the means for its accomplishment are wellknown to those skilled in the art and I prefer to illustrate one of themeans used because it assists in the description of my invention but Iwish it understood that my invention can also be applied to machines notprovided with means for improving commutation. The undesirable torquesin the commutated winding cause the frequency changer speed and hencethe frequency of the voltage supplied by it to vary with its load. Thisresults in a serious disadvantage because it is desirable that thefrequency changer supply a substantially constant frequency during itsload range. This serious disadvantage existed in frequency changers asheretfore constructed. In accordance with my invention this disadvantageis avoided by employing torque neutralizing windings having their axesapproximately midway between the commutation zones.

The following theoretical discussion will be readily appreciated bythose skilled in the art and will be of assistance in understanding myinvention. If the primaryof an alternating current dynamo electricmachine contains a commutator connected to a winding which is eitherconductively or inductively related to the primary winding, then analternating current can be taken from the commutator if the machine runsabove or below its synchronous speed and the frequency of thisalternating current will be equal to that percentage of the primaryfrequency that the slip of the machine bears to'its synchronous speed.It is evident that equal frequencies will. exist if the slips above andbelow synchron ous speed are equal, but it is often desirable to operatethe machine below synchronous speed to decrease the mechanical stressesand the commutator peripheral speed. In an alternating current dynamoelectric machine having a commutated winding and a three phase brushsystem, the numberof commutating zones per pair of magnetic poles is ingeneral six, but as so aptly developed by Scherbius, this can be reducedto three if the commutated winding has a coil pitch of 120 electricaldegrees and advantage is taken of this fact to economize and simplifythe machine.

My invntion as applied. to a frequency changer will be best understoodfrom the following description when considered in connection with theaccompanying drawings while the features of my invention which arebelieved to be novel and patentable are pointed out in the claimsappended hereto.

Fig. 1 represents a preferred embodiment of circuit connectionsemploying current commutation, voltage commutation, and torqueneutralizing windings in a self-propelled frequency changer runningbelow its synchronous speed and operated by threephase, cycle,alternating current and sup plying three-phase, 5 cycle, alternatingcurrent which I have found gives highly satisfactory commutation andstable speed operation; Fig. 2 represents the peripheral values of theampere turns of phases I, II, III of the commutated winding of themachine represented in Fig. 1; Fig. 3 represents a vector diagram methodfor determining the resultant value of ampere turns at any point on theperiphery of the commutated winding; Fig. 4 represents the flux areasproduced by the unequal peripheral ampere turn distribution of theoommutated winding and the neutralizing flux areas produced by the novelmethods of my invention. Fig. 5 illustrates the relation between thevalues of the current flowing in the respective phase windings and thecurrent flowing outward through the brushes to the low frequency loadlines; Fig. 6 represents a vector diagram method for determining theproper phases to which the commutating windings of any commutation zoneand the neutralizing windings between the commutation zones should'beconnected so as to produce the neutralizing flux areas.

To simplify, I have represented in Fig. 1, a two-pole machine with threecommutation zones but it will be readily appreciated that my inventionis equallyapplicable to any even number of poles by simply adding threecommutation zones for'each additional pair of poles and making similaror equivalent circuit connections as shown in the respective figures.

Referring to Fig. 1, 10 represents the rotatable primary winding; 11, 12and 13 respectively represent the collector rings, brushes andrelatively high frequency input lines for the primary winding; 14represents the commutated winding having a coil pitch of 120 electricaldegrees, the winding being represented as a Gramme ring forsimplification but which in practice may be of any well knownconstruction; 15, 16 and 17 represent the three sets of brushes in thethree commutation zones resting on the commutator connected to thecommutated winding, but for simplification I have omitted the commutatorand show-the brushes resting directly on the winding; 18 represents the:stationary stator secondary winding provided to make the machineself-propelling as a shunt alternating current commutator motor, thiswinding being connected to the brushes of the commutated winding and byselecting the I proper ratio of their number of turns I obtain thedesired speed; 19 represents the lines of the relatively low frequencycurrent output; 20 and 23 represent commutating windings on the statorwhich affect thecommutation zones for producing good currentcommutation; 21 represents interpole windings on the stator, alsoaffecting the commutation zones for producing good voltage commutation;22 represents adjustable or fixed stabilizing impedances which may beand preferably are for this modification pure resistances; 24 and 25represent torque neutralizing windings situated on the statorapproximate ly midway between the commutation zones.

My invention is believed to be of general application and will apply toany number of commutation zones per pair of poles but as previouslydescribed three commutation zones per pair of poles will usually be usedand for this reason and also for simplification I have illustrated myinvention as applied to a two pole machine'having three commutationzones. It therefore follows that the commutated winding will have a coilpitch of 120 electrical degrees, this being equal to the brush span asshown by the brushes 15, 16 and 17 in Fig. 2. As I have assumed athree-phase commutated winding, therefore each phase will have an ampereturn belt of 240 electrical degrees as shown by AT. AT and AT in Fig. 2.By inspection of'Fig. 2, it can be seen that we have a varying ampereturn distribution around the periphery of the commutated winding. At anypoint on the periphery such as DD in Fig. 2 the ampere turn values ofphases I and II may be represented as A and B respectively. Theresultant ampere turnvalue which we will call 0 is given by the formulaC /A' B 2AB cosine e,

as shown vectorially in Fig. 3. \Vhen the resultant C of Fig. 3 isevaluated from point to point on the periphery a curve similar to X X"in Fig. 4 will be obtained. For a sinusoidal distribution of ampereturns the numerical values at different points on the periphery shouldall be equal as on the line M M in Fig. 4. The mean value M M" will beneutralized by the flux of the relatively high frequency current flowingin the primary winding this being like the well known phenomena ofrotary converters. When the commutated winding 14 has the mean value ofits ampere turn distribution represented by M M", there is noundesirable torque produced by the winding and the frequency changerruns at a substantially constant speed during its load range. Inspectionof curve X X" in Fig. 4 shows that areas C, C and I are so related toeach other that if they are unmolested the mean "alue of the ampereturndistribution of the commutated winding would be represented by M M.The areas C, C are in the commutation zones and will induce a voltagetending to interfere with the change in the current flowing inthecommutated conductors as they move from one phase belt to another. Thephenomena relating to this rapid current change is commonly termedcurrent commutation and to this definition I will adhere throughout thisapplication. To obtain good current commutation I provide at eachcommutation zone one or more commutating windings carrying currents ofsuch phase relations and strength that their resulting flux produces theareas C" and C. These are slightly larger than the areas C and C so asto neutralize C and C and in addition provide a slight flux in thereverse sense of that of C and C, so as to induce a. voltage in the coilundergoing commutation equal and opposite to the self-induced voltagecaused in it by the change in direction or magnitude of current in itwhen undergoing commutation. These commutating windings are connected inseries with the low frequency load so that the areas C and C will varywith the load in the same manner as the areas G and C vary with the lowfrequency load. In Fig. 1, I have shown in each commutation zone twosuch commutating windings represented by 20 and 23.

The following description teaches how to determine the proper phases towhich the torque neutralizing and commutating windings should beconnected and for illustration I will now describe how the latter are tobe connected to obtain the proper phase relation for producing areas C.In Fig. 5, A and B represent two arbitrary points in any electrical.circuit, 14 represents the commuta-tor of the frequency changer and thebrushes resting on the commutator are represented by 15, 16 and 17. Inthe electrical art it is often necessary to determine the resultantvalue and direction of two or more currents flowing through one line andtherefore it is necessary to know the actual value and relativedirection of the various currents. If a current of plus 100 amperes(+100 amperes) is flowing from A to B then the electrical art regardsthe current flowing from B to A as minus 100 amperes (100 amperes). Forillustrative purposes I am assuming that the currents in the phasewindings of the commutated winding are 100 amperes and are flowing inthe direction indicated by the arrows and that I wish to determine theresultant current flowing outward through the brush 17 As the currentflowing in that phase winding represented between the brushes 15 and 17is 100 amperes and the current flows in direction from 15 to 17,therefore that current will flow outward through 17 at a value of plus100 amperes (+100 amperes). Also as the current flowing in that phasewinding represented between the brushes 17 and 16 is 100 amperes and thecurrent flows in direction from 17 to 16, therefore that current mustflow inwards through the brush 17 at a value of plus 100 amperes (+100amperes). But since I am determining the outward flow of current throughthe brush 17, therefore that last mentioned current is regarded asflowing outward through 17 at a value of minus 100 amperes (100amperes). The first thought might be that the +100 amperes and the 100amperes balance each other, resulting in zero current flowing outwardthrough 17, but the currents do not balance each other because at anyparticular instant their values are not the same due to the phase anglebetween them as is evident from the following description referring toFig. 6. Let AT, AT and AT represent the ampere turns and the currents inthe three phases of the commutated winding, they being 120 electricaldegrees apart. For illustration, assume it is desired to produce thearea C at the commutation zone of the brush 17 in Fig. 2. At brush 17the ampere turns are represented by AT, the other two phases having zeroampere turns. Now, to balance AT I connect one commutating winding 20 tobrush 17 which has outflowing currents P and P producing the vectorresultant current V as shown in Fig. 6. I connect the other commutatingwindings 23 to brush 15 which has outflowing currents +P and P producingthe vector resultant current V as shown in F 6. By reversing V andcombining it with V we get the final resultant V as shown in Fig. 6 andit is evident that by varying the number of turns in these commutatingwindings the vector V can be made of the proper magnitude to balance AT.The commutating windings 20 and 23 therefore produce flux areas C whichneutralize iiux areas C thus resulting in good current commutation.

But neutralization of flux areas C causes the flux of the commutatedwinding 14 to have a mean value which is less than the value of M .I,thus resulting in a torque between the rotor and the stator of themachine. This torque is a function of the load and is thereforeundesirable because if left unneutralized it would tend to alter thespeed with changes in load and this is usually undesirable in a machineof this type. To neutralize this torque I provide one or more torqueneutralizing windings on the stator having their axes approximatelymidway between the commutation zones. These torque neutralizing coilscarry currents of such phase relations and strength that their resultingmagnetomotive force assists the magnetomotive force produced by thewinding 141 between the commutation zones. The mag netomotive force ofthe torque neutralizing windings produces the flux area I which equalsthe flux area I and since the magnetomotive force producing I assiststhe magnetomotive force of the commutated winding the resultant flux issinusoidal as represented by M M. It follows that winding 14; will notdevelop undesirable torques and the frequency changer speed will besubstantially constant during its entire load range. These coils areconnected in series with the low frequency load so that the area I willvary with the low frequency load in the manner as the area I varies withthe low frequency load. The phases to which the torque neutralizingcoils should be connected can be determined by a method analogous to themethod illustrated in Fig. 6 and by varying the number of turns the areaI" can be produced. In Fig. 1 I have shown two torque neutralizing coils2st and 25 approximately midway between the commutation zones. I wish itunderstood however that my invention is not limited to two such windingssince it will be obvious to those skilled in the art that a differentnumber than two may be used in various modifications of the invention.

It is well known to those skilled in the art that in an alternatingcurrent commutator machine the working fiux of the machine induces avoltage and resulting current in the coil undergoing commutation bytransformer action and sparking may thus result at the commutatorbrushes. The elimination of the above sparking is commonly termedvoltage commutation and to this definition I will adhere throughout thisapplication. To obtain good voltage commutation I provide at eachcommutation zone one or more interpole windings on the stator preferablyin series with fixed or adjustable impedances. These interpole windingsare connected across the low frequency load lines and carry currents ofsuch strength and phase relation that their resulting flux neutralizesthe working flux of the machine in the commutation zone and in additionbuilds up aflux equal to of the working flux for the assumed case of a(30 cycle to 5 cycle frequency changer. This additional flux is in thereverse sense of the working flux and its function is to induce in thecoil undergoing commutation a voltage approximately equal and oppositeto that produced by the transformer action of the working flux of themachine which is of 5 cycle frequency in. the coil undergoingcommutation and which is fixed in space. The rate of transformation ofthis 5 cycle flux will be of the 60 cycle working fluX and hence tobalance this transformer voltage by means of a rotation voltage with themachine running of its synchronous speed it is necessary to have thedensity of the reverse flux equal. to

of the working flux density. The function of the stabilizing impedancesis to s ab lize the current through the interpole windings and theirmanner of operation is so well known to the art as not to requirefurther description. The phases to which these interpole windings shouldbe connected can be determined by a method analogous to the'methodillustrated in Fig. 6. In Fig. 1 I have shown one interpole winding 21in series with a stabilizing resistance 22 influencing each commutationzone.

\Vhile I have described my invention in connection with a self-propelledfrequency changer having a three-phase, 60 cycle input; three-phase, 5cycle output; two poles and three commutation zones; with both primaryand. secondary windings connected in star; with a rotatable primary andstationary secondary and employing separate windings for the primary andcommutated windings, yet I wish it to be understood that the broad scopeof my invention will be equally applicable to any alternating currentdynamo electric machine having a commutated winding irrespective of thenumber of phases or frequency of the input or output, number of poles orcommutation zones, method of connections of either primary or secondary,which member of the machine is to be rotatable and whether a common orseparate windings are employed for the primary and commutated windings,and therefore I do not wish to limit my invention to the particulararrangement herein described.

In accordance with the provisions of the patent statutes, I havedescribed the principles of operation of my invention together with theapparatus which I now consider to represent the best embodiment thereof,but I desire to have it understood that such other modifications as fallfairly within the true spirit and scope of my invention are intended tobe included within the scope of the appended claims.

lVhat I claim as new and desire to secure by Letters Patent of theUnited States, is 1- 1. A dynamo electric machine comprising relativelyrotatable members, a commutated winding on one of said members having anon-sinusoidal ampere turn distribution, brushes cooperating with thecommutator, leads connected to said brushes for carrying the loadcurrent of said commutated winding, and winding means for said othermember energized by a current whose mag nitude is substantially directlyproportional to the current flowing in said leads for producing betweenadjacent commutation zones in said other member a flux which combineswith the flux produced by the commutated winding between said zones togive a resultant flux which is substantially equivalent to the flux thatwould be produced between said zones by a eommutated winding having asinusoidal ampere turn distribution.

2. A dynamo electric machine comprising relatively rotatable members, acommutated winding on one of said members having a nonsinusoidal ampereturn distribution, brushes cooperating with the commutator, leadsconnected to said brushes for carrying the load current of saidcommutated winding, at least one winding situated with its axisapproximately midway between adjacent commutation zones on said othermember, and means for connecting said last mentioned winding in serieswith one of said leads so that the current flowing through said lastmentioned winding produces in said other member a flux which combineswith the flux produced by the commutated winding between said zones togive a resultant flux which is substantially equivalent to the flux thatwould be produced between said zones by a commutated winding having asinusoidal ampere turn distribution.

3. A dynamo electric machine comprising relatively rotatable members, acommutated winding on one of said members having a nonsinusoidal ampereturn distribution, brushes cooperating with the commutator, leadsconnected to said brushes for carrying the load current of saidcommutated winding, two windings situated with their axes approximatelymidway between adjacent commutation zones on said other member, andmeans for connecting each of said last mentioned windings in series witha different one of said leads so that the currents flowing through saidlast mentioned windings produce in said other member a flux whichcombines with the flux produced by the commutated winding between saidzones to give a resultant flux which is substantiallv equivalent to theflux that would be produced between said zones by a commutated windinghaving a sinusoidal ampere turn distribution.

4. A self propelled frequency changer of the commutator tvpe having aprimary rotor member provided with a commutated wind ing havingnonsinusoidal ampere turn dis tribution. brushes resting on thecommutator. low frequency load lines supplied from said brushes, asecondarv stator winding connected across said brushes for causing saidmachine to be self propelling, voltage commutation windings on thestator adjacent to each commutation zone and connected across thebrushes, a pair of current com mutation windings on the stator adjacentto each commutation zone connected in series with different loadcircuits, and a pair of torque neutralizing windings on the statorsituated approximately midway between each commutation zone andconnected in series with different load circuits.

In witness whereof, I have hereunto set my hand this 19th day ofNovember. 1929.

7 JOHN I. HULL;

